During the last two decades, the increasing demand for communication bandwidth boosted the development of fiber-optical communications. A number of methods dedicated to increase the capacity and bandwidth are proposed, in which dense wavelength division multiplexing (DWDM) is attractive and has been exploited due to its flexibility to upgrade or reconstruct on the basis of existing infrastructures.
DWDM technology can be used to increase the transmission capacity of a system by increasing the number of wavelengths. For instance, 80 or 100 channels with different signals can be transmitted through a single fiber at the same time, and then can be separated when they reach the subscriber users. It considerably improves the efficiency of the transmission systems. In conventional DWDM systems, distributed feedback (DFB) semiconductor lasers are the major light sources. Multiple signals are required to be transmitted simultaneously, and the wavelength interval between the lasers has to meet the International Telecommunication Union (ITU-T) standards. If semiconductor lasers with specific wavelengths are used, each subscriber user has to be equipped with a specific transceiver, which is used to download multiple wavelength signals from the central office, and then demodulate signal as needed. In order to ensure the stability of the system, a backup for each transceiver has to be configured in case of the laser failure, so the energy consumption and maintenance costs are increased. Therefore, it is desirable to use tunable lasers in DWDM systems, and then only one tunable laser backup is needed, which can be used to replace any of the wavelengths, so the complexity of the DWDM systems and the running costs are greatly reduced.
For the tunable lasers used in the central office, high performances such as narrow linewidth, wide tuning range, high stability, good reliability in harsh environment, and long life time are needed. However, the required number of lasers is small, so it's not sensitive to the price. If the tunable lasers are used in access networks such as WDM-PON, each subscriber user has to be equipped with an optical transceiver, so a large amount of tunable lasers with very low price are needed. The target price of such an optical transceiver module is $ 50, normally does not exceed $ 100, but the current market price of the basic tunable lasers are more than $ 1,000. Accordingly, the present invention is to design and produce low-cost tunable lasers for the majority of WDM-PON market. Fast tuning speed is not required in WDM-PON systems, but the tuning range should be larger than 10 nm so as to cover multiple ITU-T 100 GHz channels.
In the 1980s, the research on tunable lasers has been started. A typical tunable laser comprises a gain region and a wavelength tunable filter, which are operated by changing the temperature, electric current, electric field, or by changing the wavelength mechanically. The structures of the tunable lasers can be divided into three major categories: external cavity laser, distributed Bragg reflector (DBR) laser, and distributed feedback (DFB) laser array. These lasers can be designed to achieve a tuning range of more than 40 nm, which can meet the requirements of DWDM systems, though they have their own advantages and disadvantages.
The performance of external cavity tunable laser is desirable. It can be continuously tuned over a wide wavelength range with high output power, and can obtain optical linewidth of less than 100 kHz. However, the packaging of the external cavity laser is complicated and costly, because the wavelength tuning component is complex.
The DBR tunable lasers are extensively studied, which use the vernier principle between two gratings to achieve a wide wavelength tuning range. The packaging of DBR laser is relatively simple comparing with the external cavity laser, but the fabrication of such a laser is difficult because the active and passive waveguide have to be integrated. So it is difficult to achieve large-scale industrial production, and therefore hinders the development of such kind of lasers. Furthermore, the wavelength of DBR lasers are tuned by adjusting the injection currents, so the tuning speed is fast but the linewidth is broadened.
The cost of DFB tunable lasers is relatively cheap, but the tuning range is small (eg. 3 nm), because the refractive index of the active material changes slowly with temperature or current. Therefore, several wavelength specific DFB lasers are used to form a DFB laser array in order to increase the tuning range. The advantages of this approach are: the performance is very stable; there is no mode hoping; the packaging and wavelength tuning configuration are relatively simple.
The researchers in China have also studied the tunable lasers in depth. For example, Professor He Jianjun's group at Zhejiang University proposed a low-cost V-shaped coupling cavity widely tunable semiconductor laser (“Simple and compact V-cavity semiconductor laser with 50×100 GHz wavelength tuning”, Vol. 21, No. 11, Optics Express, 2013). It has 50 channels with 100 GHz wavelength spacing covering the entire C-band.
More specifically, the external tunable lasers can be referred to the research results of Intel (“Automated Optical Packaging Technology for 10 Gb/s Transceivers and its Application to a Low-Cost Full C-Band Tunable Transmitter,” Intel Technology Journal, vol. 08, 101-114, 2004.) and NEC (“Full C-Band External Cavity Wavelength Tunable Laser Using a Liquid-Crystal-Based Tunable Mirror,” IEEE Phton. Tech. Lett., vol. 17, 681-683, 2005.). The DBR lasers can be referred to the research results of JDSU (“Tunable Semiconductor Lasers: A Tutorial,” J. Lightwave Technol., vol. 22, 193-202, 2004.), Oclaro (“Widely Tunable DS-DBR Laser With Monolithically Integrated SOA: Design and Performance,” IEEE J. Select. Topics Quantum Electron., vol. 11, 149-156, 2005.), and Syntune (Jan-Olof Wesström, Stefan Hammerfeldt, Jens Buus, Robert Siljan, Reinhard Laroy, and Harry de Vries, “Design of a Widely Tunable Modulated Grating Y-branch Laser using the Additive Vernier Effect for Improved Super-Mode Selection”). The DFB laser arrays can be referred to the research result of NEC (“Wavelength-Selectable microarray light sources for S-, C-, and L-band WDM systems,” IEEE Photon. Technol. Lett., vol. 15, 903-905, 2003.). It covers the S, C and L communication bands with six eight-wavelength DFB laser arrays.
However, the realization of lasers with different wavelengths on the same chip is not easy. The traditional method is using electron beam lithography (EBL) to fabricate different gratings. However, the EBL costs high, and it's time consuming. In addition, due to the limited writing field of EBL, it's not suitable to fabricate devices in a large area. To solve these problems, Professor Xianfei Chen at Nanjing University filed an international PCT patent (Application No. PCT/CN2007/000601). Holographic exposure and conventional photolithographic are used to fabricate the Bragg gratings, so the DFB lasers with different lasing wavelengths can be fabricated on the same wafer, which greatly reduces the fabrication time and is easy to implement large scale production, thereby further reduces the overall cost. Some research results concerning the REC technique details and DFB lasers and laser arrays can be found in Chinese invention patent “Method and apparatus for manufacturing monolithic semiconductor laser array” (CN200810156592.0) and the literature: Jingsi Li, Huan Wang, Xiangfei Chen, et. al, “Experimental demonstration of distributed feedback semiconductor lasers based on reconstruction-equivalent-chirp technology”, Optics Express, 2009, 17 (7): 5240-5245, and Yuechun Shi, Xiangfei Chen, et. al, “Experimental demonstration of eight-wavelength distributed feedback semiconductor laser array using equivalent phase shift”, Optics Letters, 2012 37 (16), p 3315-3317.
The present invention provides series and hybrid series/parallel integration approaches to design and fabricate tunable DFB lasers based on REC technique. The wavelength tuning is achieved by changing the temperature and the injection currents.